From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 189EEC67871 for ; Thu, 27 Oct 2022 08:54:49 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 4D7B28E0002; Thu, 27 Oct 2022 04:54:49 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 487428E0001; Thu, 27 Oct 2022 04:54:49 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 3773D8E0002; Thu, 27 Oct 2022 04:54:49 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0014.hostedemail.com [216.40.44.14]) by kanga.kvack.org (Postfix) with ESMTP id 2433F8E0001 for ; Thu, 27 Oct 2022 04:54:49 -0400 (EDT) Received: from smtpin07.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay03.hostedemail.com (Postfix) with ESMTP id 5F688A094E for ; Thu, 27 Oct 2022 08:54:48 +0000 (UTC) X-FDA: 80066119056.07.27C2AF5 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by imf11.hostedemail.com (Postfix) with ESMTP id ECC7C4003A for ; Thu, 27 Oct 2022 08:54:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1666860886; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=9TI2B7hbrnS5d40DmcnVyemiw5/MhD+gxVcp0Tq27Ec=; b=YpKbq0QAK8PGdDup2kCcC/z//pLXGJMsksFb4xg3xgjFx8+sdxREQDkrUX3Kdt/VguG2sI F2xE6MVWG/12gvg8StLSphIwm9XECMuRyWuH39FRaid+fbV6ibuKMVassA9CvkA0OTrSEl DA3l6P4gZInFvdVq9Je2FFpob56g95E= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-335-mGBDuEgjPtaro3w9rOCEMA-1; Thu, 27 Oct 2022 04:54:43 -0400 X-MC-Unique: mGBDuEgjPtaro3w9rOCEMA-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 956088027EC; Thu, 27 Oct 2022 08:54:42 +0000 (UTC) Received: from localhost (ovpn-12-121.pek2.redhat.com [10.72.12.121]) by smtp.corp.redhat.com (Postfix) with ESMTPS id DD0E7492B07; Thu, 27 Oct 2022 08:54:41 +0000 (UTC) Date: Thu, 27 Oct 2022 16:54:38 +0800 From: Baoquan He To: Stephen Brennan Cc: Andrew Morton , linux-mm@kvack.org, Uladzislau Rezki , Christoph Hellwig , Matthew Wilcox Subject: Re: /proc/kcore reads 0's for vmap_block Message-ID: References: <87ilk6gos2.fsf@oracle.com> MIME-Version: 1.0 In-Reply-To: <87ilk6gos2.fsf@oracle.com> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.10 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1666860888; a=rsa-sha256; cv=none; b=ZieeDbkUHCp0uMkA1xpGvycrZSG+s1fk+1v6knvI5m3rW0tDnzuqOUsjDMv5VgAemyblrd 80xowKLDbm0s/dhCGnFhIUXLBRdEDyKIMpVKfUfsLUiDtSmKkL43SsHHvZp+6lFN4lpH3v 6u1KjP1tLPTpy1vJinVn7U+95Du9TOw= ARC-Authentication-Results: i=1; imf11.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=YpKbq0QA; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf11.hostedemail.com: domain of bhe@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=bhe@redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1666860888; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=9TI2B7hbrnS5d40DmcnVyemiw5/MhD+gxVcp0Tq27Ec=; b=CDyeJaQMGkQPFFd82MWoAxNyL3mPXEeBxDQY5KHH3tjBe2Vs0Af4mFzIZ1BFX7o7LP9m2l q6lu/NItkb44UsQQl+v91m4IL7o40QXlxcFfB5S0ll4cCdnqVCbPdB9RF1cltPoiS6WFzZ OvSr9u8t0hoKo5uRVyVYBvUF3DwuHHI= X-Rspamd-Queue-Id: ECC7C4003A Authentication-Results: imf11.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=YpKbq0QA; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf11.hostedemail.com: domain of bhe@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=bhe@redhat.com X-Rspamd-Server: rspam12 X-Rspam-User: X-Stat-Signature: tbnqe51bcawkhwr46r8myzjmg3pnza4j X-HE-Tag: 1666860886-92831 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 10/26/22 at 04:15pm, Stephen Brennan wrote: > Hi all, > > The /proc/kcore interface uses vread() to read memory addresses which > are in the vmalloc region, and it seems that vread() doesn't support > vmap_areas which are associated with a vmap_block. As far as I can tell, > those have vmap_area.vm == NULL, and get skipped during vread()'s > iteration. So the end result is that the read simply returns 0's. Hmm, with my understanding, it should be vm_map_ram() which is called without a struct vm_struct associated, and it's the only interface do to so. vmap_block is the optimization way based on percpu to reduce vmap lock race. > > This impacts live debuggers like gdb and drgn, which is how I stumbled > upon it[1]. It looks like crash avoids the issue by doing a page table > walk and reading the physical address. > > I'm wondering if there's any rationale for this omission from vread(): > is it a simple oversight, or was it omitted due to the difficulty? Is > it possible for /proc/kcore to simply take the page faults when it reads > unmapped memory and handle them? (I'm sure that's already discussed or > is obviously infeasible for some reason beyond me.) >From git history, the vmlist iterating was taken in vread() at the first place. Later, in below commit, when people changed to iterate vmap_area_list instead, they just inherited the old code logic. I guess that's why vmap with NULL ->vm is skipped. commit e81ce85f960c ("mm, vmalloc: iterate vmap_area_list, instead of vmlist in vread/vwrite()") > > Ideally, I'm just looking for a way forward that allows the debugger to > *work* as expected, meaning either that /proc/kcore always reads the > correct data, or that the debugger can know ahead of time that it will > need to do some processing (like a page table walk) first. I think we can adjust vread() to allow those vmap_area with NULL ->vm being read out? Made a draft patch at below, please feel free to have a test. Not sure if there's any risk. >From 9f1b786730f3ee0a8d5b48a94dbefa674102d7b9 Mon Sep 17 00:00:00 2001 From: Baoquan He Date: Thu, 27 Oct 2022 16:20:26 +0800 Subject: [PATCH] mm/vmalloc.c: allow to read out vm_map_ram() areas in vread() Content-type: text/plain Currently, vread can read out vmalloc areas who is associated with a vm_struct. While this doesn't work for areas created by vm_map_ram() interface because it doesn't allocate a vm_struct. Then in vread(), these areas will be skipped. Pages are passed into vm_map_ram() and mapped onto frea vmap area, it should be safe to read them out. Change code to allow to read out these vm_map_ram() areas in vread(). Signed-off-by: Baoquan He --- mm/vmalloc.c | 15 +++++++-------- 1 file changed, 7 insertions(+), 8 deletions(-) diff --git a/mm/vmalloc.c b/mm/vmalloc.c index ccaa461998f3..f899ab784671 100644 --- a/mm/vmalloc.c +++ b/mm/vmalloc.c @@ -3526,7 +3526,7 @@ long vread(char *buf, char *addr, unsigned long count) struct vm_struct *vm; char *vaddr, *buf_start = buf; unsigned long buflen = count; - unsigned long n; + unsigned long n, size; addr = kasan_reset_tag(addr); @@ -3547,12 +3547,11 @@ long vread(char *buf, char *addr, unsigned long count) if (!count) break; - if (!va->vm) - continue; - vm = va->vm; - vaddr = (char *) vm->addr; - if (addr >= vaddr + get_vm_area_size(vm)) + vaddr = (char *) va->va_start; + size = vm ? get_vm_area_size(vm) : va_size(va); + + if (addr >= vaddr + size) continue; while (addr < vaddr) { if (count == 0) @@ -3562,10 +3561,10 @@ long vread(char *buf, char *addr, unsigned long count) addr++; count--; } - n = vaddr + get_vm_area_size(vm) - addr; + n = vaddr + size - addr; if (n > count) n = count; - if (!(vm->flags & VM_IOREMAP)) + if (!vm || !(vm->flags & VM_IOREMAP)) aligned_vread(buf, addr, n); else /* IOREMAP area is treated as memory hole */ memset(buf, 0, n); -- 2.34.1